Steam condensers are fundamental components of about 85% of electricity generation plants, and about 50% of the desalination plants installed globally. As a consequence, finding routes that even moderately improve efficiency of the condensation process could lead to considerable economic savings as well as environmental and societal benefits.
Since the 1930s, the hydrophobization of metal surfaces has been known to increase heat transfer during water condensation by up to an order of magnitude. This surface modification switches the condensation mode from filmwise (FWC) to dropwise (DWC). However, the use of hydrophobic coatings required to promote DWC introduces an additional resistance to heat flow. Thus, in simplified terms, to increase the total heat transfer rate, thermal resistance introduced by the hydrophobic coating must be significantly smaller than that posed by the water film during condensation.
While there are many techniques to render surfaces hydrophobic to promote DWC, most conventional coatings suffer from longevity issues. Moreover, in addition to their limited durability, most hydrophobic surface modifiers have a low thermal conductivity, preventing the widespread industrial adoption of the condensation mode. For example, to withstand the steam environment within a power plant condenser during the projected lifetime of the power station (about 40 years), it is estimated that a Polytetrafluoroethylene (PTFE) film must be about 20 to 30 μm thick, where the thermal resistance added by this thickness of the polymeric film negates any heat transfer enhancement attained by promoting DWC, (see for example J. W. Rose, Proc. Inst. Mech. Eng., A 2002, 216, 115.)
Recently, several alternative durable hydrophobic materials have been proposed including rare earth oxides, grafted polymers, and lubricant impregnated surfaces (LIS). Nevertheless, applying these materials as thin films makes them susceptible to variety of degradation issues including polymer oxidation at defect sites, ceramic film delamination due to thermal expansion coefficient mismatch between the film and underlying metal, and, for LIS, slow lubricant drainage with departing water drops.
Metal matrix composites with hydrophobic particles have been proposed as a durable alternative to thin film hydrophobic surface coatings. In particular, polished copper-graphite microparticle composites have been shown to have a macroscopic water drop contact angle of about 87° (see for example M. Nosonovsky, V. Hejazi, A. E. Nyong, P. K. Rohatgi, Langmuir 2011, 27, 14419.) The surface of this composite has heterogeneous wetting properties consisting of microscale hydrophobic patches on a hydrophilic background. Condensation and wetting on surfaces with microscale chemical and topological heterogeneities has been studied extensively, and surfaces comparable to those of the composites with microscale hydrophobic features have been demonstrated to flood during condensation. This mismatch between macroscale wetting properties and condensation mode stems from the multiscale nature of the phase change process. In practical terms, flooding of surfaces with microscale hydrophobic features occurs because microdroplets smaller than the features nucleate, grow, and coalesce into a film on the hydrophilic background surface surrounding the hydrophobic phase.
The flooding of composite surfaces during condensation can be prevented by engineering the materials on length scale greater than that of drop nuclei but significantly smaller than the average separation distance between microdroplet centers prior to onset of the coalescence dominated growth stage of about 5 to 10 μm.